Load cell

ABSTRACT

A load cell comprising a load carrying structural framework is arranged with at least one measuring zone ( 15, 16 ) arranged with a measuring means. A load F applied in the x-direction parallel to two outer beams ( 1, 4 ) of the framework is transferred to two inner beams ( 2, 3 ) by two side beams ( 5, 6 ). The load on the measuring zones ( 15, 16 ) is greater than the load applied parallel to outer beams ( 1, 4 ) in a ratio of the distance h between the joints of the two outer beams divided by the distance d between the joints of the two inner beams. When a load is applied in the y-direction parallel to the two side beams ( 5, 6 ), the load on the measuring zones is also greater than the load applied at the side beams, in this case by a ratio of the distance l between the joints ( 7  and  11 ), ( 10  and  14 ) divided by distance d. In a load cell according to the preferred embodiment of the invention measuring zones ( 315, 316 ) are defined by recesses  330-337  and planar gaps  317-319  running from one side of the load cell to the other in the z-direction. The load cell according to the invention is accurate even with small loads and is insensitive to the effects of temperature gradients across the load cell in the x, y, or z-direction.

TECHNICAL FIELD

The present invention relates to a measurement of force for industrialapplications, and more particularly to load cells for continuousmeasurement of forces associated with weight or pressure, including themeasurement of tension in web or sheet materials.

BACKGROUND ART

Within many industrial areas it is necessary to measure accurately themagnitude of a force. An example of a load cell for the measurement of aforce in a horizontal direction is known.

A load cell described in U.S. Pat. No. 5,250,762 is arranged with twoparallel beam. The two parallel beams are joined by side beams, makingup a four sided frame, and the two parallel beams are also joined by oneor more measuring zones. As a result of a load F applied to the loadcell in a direction parallel to the two parallel beams, a shear force isapplied to the one or more measuring zones joining the two parallelbeams. The shear force in the measuring zones is measured by amagnetoelastic sensor of the Pressductor type.

This load cell has a long service life, tolerates overloads well andperforms accurately in service. However, this load cell requires acertain minimum load in order to generate a useable measurement signal.

SUMMARY OF THE INVENTION

The objects of the invention include that of providing a load cell thataccurately measures an applied load. A further object of the inventionis to accurately measure a component of a load when such a loadcomponent is parallel to an outer beam of the load cell. A still furtherobject of the invention is to provide a load cell that measures smallloads accurately. Another object of the invention is to provide a loadcell in which the effects of temperature gradients across the load cellare minimised. Another further object is to minimise the influence onthe load cell from any load other than the measured load. The objectsalso include that of providing a load cell that produces measurementsignal with a high output level at small loads, compared to the priorart.

A load cell according to the invention comprises a structuralload-carrying framework of beams and joints arranged with one or moremeasuring zones. The structural load-carrying framework is arranged sothat a load applied to an outer beam of the framework is transferred toa second and outer beam via two side beams, which side beams are alsoconnected to two inner beams which are further connected to each otherby at least one measuring zone.

The structural load-carrying framework can instead be loaded by means ofone of the side beams, in which case the load is transferred to thesecond side beam via the outer and inner beams.

A load cell according to a preferred embodiment of the present inventionis shaped externally as a solid and substantially rectangular block.Inside the substantially rectangular block the load cell is furthershaped such that the load applied in an x-direction parallel to twosubstantially parallel outer beams of the load cell is transferred andamplified by two side beams connecting the outer beams to twosubstantially parallel inner beams, which inner beams are connectedtogether by two measuring zones on each of which a measuring means isarranged. Each of the measuring zones equipped with measuring means issubjected to a shear load in the direction of the long axis of the loadcell. The load transferred to the two measuring zones is greater thanthe external load on the outer part of the load cell. Through this typeof leverage action relatively small loads may be measured accurately.The leverage or amplification of the applied force, when the force isapplied in the x-direction parallel to the two outer beams oriented inthe long axis of the load cell, is in proportion to the ratio of thedistance h between the outer beams, divided by the distance d betweenthe inner beams. More specifically h is the distance between joints 7and 10, 11 and 14 of the outer beams 1, 4, and d is the distance betweenthe joints 8 and 9, 12 and 13 of the inner beams 2, 3.

In case where the applied load is applied to the side beams (5, 6) in ay-direction parallel to the side beams, the leverage or amplification isin proportion to a ratio of the dimension 1, the length of the outerbeams 1, 4 as defined by the distance between joints 7 and 11, 10 and14, divided by the distance d between the joints 8 and 9, 12 and 13 ofthe inner beams 2, 3.

A measuring means is arranged on each measuring zone to generate asignal proportional to the mechanical strain imposed on the measuringzone. A preferred embodiment of the invention comprises two measuringareas where a measuring signal is taken from a measuring zone arrangedon each area. The advantage of the present invention is that leverage oramplification due to the shape and geometry of the load cell makes itsensitive to small loads. As a result of that, a signal with a highoutput level is achieved for a relatively small load applied to the loadcell, in comparison to the prior art. By this means, a useable signal isproduced under small loads.

Another advantage is that the two beams arranged with measuring zonesare joined and arranged with a geometry that makes the load cellrelatively insensitive to a temperature gradient in the x-direction.This is because that temperature gradient results in stresses in they-direction on the measuring zones, and the measuring zones areinsensitive to stresses in that direction. The load cell is alsoinsensitive to a temperature gradient in the y-direction. This isbecause that temperature gradient gives rise to shear forces over themeasuring zones. These shear forces would, however, be opposite in signand thereby be cancelled electrically in the output signals frommeasuring means arranged on the two measuring zones. A temperaturegradient in the z-direction produces no effect on measuring zones thatare located symmetrically in the load cell.

A still further advantage of the invention is that deformation underload is extremely small. A yet still further advantage includes that agiven size of load cell according to the invention may thus be used tomeasure a wider range of loads by selecting suitable ratios of thequotient h/d, or by selecting a suitable ratio of l/d. An additionalfurther advantage of the invention is that the shape and geometry makesthe load cell insensitive to loads perpendicular to the measured load.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in more detail in connectionwith the enclosed drawings.

FIG. 1 shows a load cell comprising a framework according to anembodiment of the present invention.

FIG. 2 a shows a load cell comprising a framework according to anembodiment of the present invention under a load applied in anx-direction.

FIG. 2 b shows a load cell comprising a framework according to anembodiment of the present invention under a load applied in ay-direction.

FIG. 3 shows an isometric view of a load cell according to an embodimentof the invention which is formed from a rectangular block.

FIG. 4 shows a view of one side of a load cell according to anembodiment of the invention.

FIG. 5 shows one side of a load cell according to an alternativeembodiment of the invention.

FIG. 6 shows one side of a load cell according to a further alternativeembodiment of the invention.

FIG. 7 shows one side of a load cell according to an embodiment of theinvention in a view in which joints are indicated with referencenumbers.

FIG. 8 shows a view of one side of a load cell according to anotherembodiment of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a load cell comprising a framework according to the presentinvention. The framework is represented by beams shown as thick linesand joints drawn as solid circles. The framework comprises beams 1-6 andjoints 7-14. There are two outer beams 1, 4. The two outer beams 1, 4are connected to two inner beams 2, 3 by side beams 5, 6 representingthe sides of the framework. Inner beams 2, 3 are connected to each otherby measuring zones 15, 16 equipped with measuring means. An arrowshowing an x-direction and a y-direction relative to the framework areincluded in FIG. 1.

FIG. 2 a shows the same load cell as shown in FIG. 1 comprising aframework according to the present invention but under a load F, a loadapplied to the outer beams in an x-direction. The two measuring zones15, 16 are each subjected to a shear force as inner beams 2, 3 moveapart from each other in the x-direction because of the load F. Themagnitude of the shear force in the measuring zones is amplified by atype of leverage effect. The amount of amplification varies according tothe ratio of the distance h between the two outer beams divided by thedistance d between the two inner beams. More specifically the distancesmay be described as the distance h between joints 7 and 10, 11 and 14 ofthe outer beams 1, 4, relative to the distance d between the joints 8and 9, 12 and 13 of the inner beams 2, 3.

A load F applied in the x-direction parallel to the outer beams 1, 4results in a force over the measuring zones equal toF*k*h/dwhere h and d are as described above.

k is a constant which is due to the non-ideal nature of the joints whichhave a stiffness.

Thus the ratio of h/d determines the amplification in the measuringareas of a load F applied in the x-direction parallel to the outer beams1, 4.

FIG. 2 b shows the same load cell as shown in FIGS. 1 and 2 a comprisinga framework according to the present invention but under a load F, aload applied in a y-direction parallel to the side beams. The load cellunder loading in the y-direction has the same form and mechanicalcharacteristics in every way as FIGS. 1, 2 a and as embodiments shown inFIGS. 3-8 with the single exception that a load F applied parallel tothe side beams 5, 6, results in a force over the measuring zones equaltoF*k*l/dwhere l is distance between the joints 7 and 11, 10 and 14 of the outerbeams and where d and k are as previously described.

FIG. 3 shows an isometric view of a load cell according to a preferredembodiment of the invention. The load cell of FIG. 3 is a substantiallyrectangular shaped block that corresponds to the load cell comprisingthe framework shown in FIGS. 1, 2 a, 2 b. The perpendicular axialdirections x, y and z referred to in this description relative to theload cell are indicated in FIG. 3.

In FIG. 3 two outer beams 301, 304 are indicated which correspond toouter beams 1, 4 of FIG. 1. Similarly numbered are inner beams 302, 303and measuring zones 315, 316 which correspond to inner beams 2, 3 andmeasuring zones 15, 16 of FIG. 1. Similarly joints 311 and 314correspond to joints 11 and 14 of FIG. 1. In addition, two shoulders320, 321 and 322, 323 are shown on each of the outer beams, and two moreshoulders are mounted on each of the side beams 305, 306.

The elements of the load cell according to the preferred embodiment areshown in detail in FIG. 3. The load cell is shaped internally such thatit comprises two measuring zones corresponding to 15, 16 of FIGS. 1, 2a, 2 b arranged with measuring means. The measuring zones 315, 316 aredefined in the solid material of the load cell by planar gaps 317, 318,319, and recesses 330-337 that define the two inner beams. The planargaps run through the solid material of the load cell in the z-direction.The recesses such as 335, 337 are shaped so as to form a narrowing orwaist in a beam such as 301, 304 which has the function of forming ajoint, shown numbered as 311, 314 in FIG. 3, equivalent to joints suchas 11, 14 in FIG. 1. The recesses are conveniently formed fromcylindrical holes bored through the solid material of the load cell inthe z-direction from one side of the load cell to the other. In thepresent invention a recess is a cavity in, or a cut-out from, the solidmaterial of a beam. Such a recess runs completely through the crosssection of the beam in the z-direction. In the preferred embodiment therecesses have a partially semi-circular cross section.

The flexible joints corresponding to 7-14 formed in the embodiment areshown with reference numbers 307-314 in a separate figure, FIG. 7, forthe sake of visual simplicity. The joints are described as flexible inas much as they permit beams such as a side beam 5, 305 and an outerbeam 1, 301 connected by a joint 7, 307 to flex towards or away fromeach other under the influence of an applied load.

Two pairs of shoulders 320, 321 and 322, 323 are shown in FIG. 3arranged on the two outer beams 301, 304 of the load cell. Theseshoulders contain means such as threaded holes such as 320 a, 320 b formounting the load cell suitably between a support and a device uponwhich a x-direction component of a load, or the total load if it isacting only in the x-direction, may be measured by the load cell.

Similarly two pairs of shoulders 324-327 are mounted on the side beams305, 306 of the load cell for mounting the load cell suitably formeasuring a y-direction component of a load, or the total load if it isacting only in the y-direction.

Examining one measuring zone in detail, referring to FIG. 4. There arefour recesses 330, 332 and 335, 337 arranged such that thin sections ofmaterial remain between the recess and the edges of the load cell. Thesethin sections and those between two separate, adjacent but not connectedrecesses such as 330, 331, of measuring zone 315, form the mechanicalequivalent of joints such as joints 7, 8, 9, 10 of FIG. 1. There arefour such joints formed by the recesses 330-332 and 335-337 thatsurround each measuring area.

The planar gaps such as planar gaps 317-319 shown in FIGS. 3, 4 joiningthe recesses and defining the measuring zones may be described as plaincuts represented by planes in the z-direction.

A measuring zone 15, 16 in the load cell according to the invention isfurther shaped in the preferred embodiment such that it is relativelythin in the z-direction in comparison to the thickness of any of thebeams 1-6, 301-306. This may be seen in the isometric view of the loadcell in FIG. 3. The shaping of the load cell is carried out using amilling process or a water jet abrading process or a combination ofthose and other suitable processes for accurately removing metal from asolid rectangular block of a suitable metal, for example a type ofstainless steel. A measuring means for strain measurement is arranged onthe thin part in the load cell that forms the measuring zones 15, 16.The measuring means is preferably a magnetoelastic sensor of thePressductor type, as indicated in the Figures, in particular in FIG. 4.

The operation of a magnetoelastic sensor of the Pressductor type isbased on the fact that the magnetic permeability of a magnetic materialchanges under mechanical stress. This type of sensor is machined in thematerial of a measuring zone in a load cell. Primary and secondarywindings are each wound in holes in the load cell material so that thewindings cross at right angles. Referring to FIG. 4. Two holes in, forexample, positions 342, 343 are provided for a primary winding formagnetisation of the material with alternating current. Two more holesin, for example, positions 340, 341 are provided for a secondary windingfor measurement of voltages that arise due to an imposed load. The holesfor windings in the magnetoelastic sensor of the Pressductor type areindicated with reference numbers in only one example, FIG. 4, for thesake of clarity.

A measuring means for strain measurement may also be arranged on themeasuring zone in the form of strain gauges or one or morepiezo-electric devices.

In another embodiment of the invention the measuring zones 15, 16 arenot reduced in thickness in the z-direction relative to the thickness ofany of the beams 1-6, 301-306. In yet another embodiment of theinvention, the measuring zones 15, 16 are formed as relatively thicksections that may be thicker in the z-direction than the beams of theload cell. In yet another embodiment of the invention at least onemeasuring zone is formed as a relatively thin measuring zone of asimilar shape but manufactured from a separate piece of material. The atleast one separate measuring zone is subsequently fixed in place, forexample by welding or bonding, although other known and suitable methodsof making a material to material attachment may be used.

An alternative embodiment is shown in FIG. 5, where one single measuringzone 15 is used to measure applied load. A further embodiment with asingle measuring zone is shown in FIG. 8. Another embodiment in whichthe lengths of side beams 5,6 may not be equal may be advantageous incertain applications, for example with embodiments having a singlemeasuring zone, of which two are shown in FIG. 5, 8. Yet anotheralternative embodiment is shown in FIG. 6. In this embodiment, jointsaround one measuring zone 15 have been formed by planar gaps 618, 619,instead forming a joint by means of a recess. The planar gaps 618 and619 correspond to the planar gaps 318, 319 of FIGS. 3 and 4 but in thisembodiment have been extended in one part of the load cell to formjoints corresponding to joints 7-10 of FIG. 1.

In a development of the invention for use in certain environments, forexample severe environments where abrasive dusts or corrosive fluidscould come in contact with the load cell, the recesses and planar gapsmay be filled with a substance. For example the load cell may bepartially coated or completely encapsulated by means of a polymeric orelastomeric coating of, for example, silicone rubber. In this case thereis a material present in at least one planar gap or recess which has amodulus of elasticity lower than that of an adjacent beam.

It is within the scope of the claims of the invention that any of therecesses formed by cylindrical openings 330-337 bored through the bodyof the solid load cell may alternatively have cross sections other thanround circles, and/or that the cylindrical openings may have differentdiameters. One or more recesses may be replaced by a planar gap, or anelongation of an existing planar gap, so forming a joint as indicated inFIG. 6. It is similarly obvious that any of the beams may alternativelyhave a shape other than that of a straight beam, for example one or moreof the two outer beams may be curved or bowed in cross-section and stillbe equivalent in function to the substantially straight beams shown inthe embodiments. It is also within the scope of the claims to substituteone or more mechanical hinges for any of the flexible jointscorresponding to joints 7-10 and 11-14 of FIG. 1.

1. A load cell comprising a load-carrying structural framework of twoessentially parallel rigid beams, arranged essentially parallel to anapplied load (f) and at least one measuring zone equipped with means forstrain measurement arranged between the two beams, wherein thestructural framework comprises two outer beams (1, 4, 301, 304) whichare connected by means of joints (7-10, 307-310) to two inner beams (2,3, 302, 303) by at least two side beams (5, 6, 305, 306) which eachextend between said two outer beams and connect to said two outer beamsand to said two inner beams, and at least one measuring zone (15, 16,315, 316) equipped with means for strain measurement is arranged betweenthe two inner beams, and upon an application of a load in an x-directionparallel to the two outer beams, the amount of load applied to said atleast one measuring zone (15, 16) is in proportion to a ratio of adistance h between the joints (7 and 10, 11 and 14, 307 and 310, 311 and314) connecting outer beams (1, 4, 301, 304) to side beams (5, 6, 305,306) divided by a distance d between the joints (8 and 9, 12 and 13, 308and 309, 312 and 313) connecting the inner beams (2, 3, 302, 303) to theside beams (5, 6, 305, 306) and said means for strain measurement aresuitably adapted such that an output signal representative of theapplied load or detected load change is generated.
 2. A load cellaccording to claim 1, wherein at least one joint (7-10, 11-14, 307-310,311-314) is formed by at least one recess (330-332, 335-337).
 3. A loadcell according to claim 2, wherein at least one inner beam comprises atleast one recess (331, 333, 334, 336).
 4. A load cell according to claim2, wherein at least one recess includes a material with a lower elasticmodulus than an adjacent beam.
 5. A load cell according to claim 1,wherein the inner beams are joined to each other by said at least onemeasuring zone.
 6. A load cell according to claim 1, wherein the twoinner beams are separated from one another along part of their innerlength by at least one planar gap (318, 618).
 7. A load cell accordingto claim 1, wherein at least one joint (7-10, 307-310) is formed by atleast one planar gap (618, 619).
 8. A load cell according to claim 1,wherein at least one said measuring zone further comprises an area ofreduced thickness relative to the thickness of any of the beams (1-6,301-306).
 9. A load cell according claim 1, wherein at least one saidmeasuring zone further comprises an area of the same thickness relativeto the thickness of any of the beams (1-6, 301-306).
 10. A load cellaccording to claim 9, wherein at least one planar gap includes amaterial with a lower elastic modulus that an adjacent beam.
 11. A loadcell according to claim 1, wherein at least one said measuring zonefurther comprises an area of increased thickness relative to thethickness of any of the beams.
 12. A load cell according to claim 1,wherein said measuring means arranged on said measuring zone (15, 16,315, 316) consists of one or more magnetoelastic sensors.
 13. A loadcell according to claim 1, wherein said measuring means arranged on saidmeasuring zone (15, 16, 315, 316) consists of one or more magnetoelasticsensors.
 14. A load cell according to claim 1, wherein said measuringmeans consists of one or more piezo-electric devices.
 15. The use of aload cell according to claim 1 for measurement of an x-directioncomponent of a load F or the total load when applied in the x-direction.16. A load cell comprising a load-carrying structural framework of twoessentially parallel rigid beams, arranged essentially perpendicular toan applied load (f) and at least one measuring zone equipped with meansfor strain measurement arranged between the two beams, wherein thestructural framework comprises two outer beams (1, 4, 301, 304) whichare connected by means of joints (7-10, 307-310) to two inner beams (2,3, 302, 303) by at least two side beams (5, 6, 305, 306) which eachextend between said two outer beams and connect to said two outer beamsand to said two inner beams, and at least one measuring zone (15, 16,315, 316) equipped with means for strain measurement is arranged betweenthe two inner beams, and upon an application of a load in a y-directionparallel to the two side beams, the amount of load applied to said atleast one measuring zone (15, 16) is in proportion to a ratio of adistance l between the joints (7 and 11, 10 and 14, 307 and 311, 310 and314) connecting outer beams (1, 4, 301, 304) to side beams (5, 6, 305,306) divided by a distance d between the joints (8, and 9, 12 and 13,308 and 309, 312 and 313) connecting the inner beams (2, 3, 302, 303) tothe side beams (5, 6, 305, 306) and said means for strain measurementare suitably adapted such that an output signal representative of theapplied load or detected load change is generated.
 17. The use of a loadcell according to claim 16, for measurement of a y-direction componentof a load F or the total load when applied in the y-direction.